Induction sealing, otherwise known as cap sealing, is a non-contact method of heating a metallic disk to hermetically seal the top of plastic and glass containers. This sealing process takes place after the container has been filled and capped.
The closure is supplied to the bottler with aluminum foil liner already inserted. A typical induction liner is multi-layered. The top layer (distanced from the container opening) is a paperpulp that is generally spot-glued to the cap. The next layer is wax that is used to bond a layer of aluminum foil to the pulp. The bottom layer is a polymer film laminated to the foil. In conventional capping techniques, after the cap is applied over the opening of the container, the container passes under an induction coil, which emits an oscillating electromagnetic field and the conductive aluminum foil liner begins heats. The heat melts the wax, which is absorbed into the pulp backing and releases the foil from the cap. The polymer film also heats and flows onto the lip of the container. When cooled, the polymer creates a bond with the container resulting in a hermetically sealed product. Neither the container nor its contents are affected, and this all happens in a matter of seconds or even less.
At times, it is possible to overheat the foil causing damage to the seal layer and to any protective barriers. This could result in faulty seals, even weeks after the initial sealing process.
Japanese unexamined patent application publication No. 2003-307505 describes a thermography based system for detecting defects in the fusion of the aluminum seal to a bottle's opening.
In addition, U.S. Pat. No. 7,434,986 describes an apparatus for monitoring and detecting sealing defects. The system described in U.S. Pat. No. 7,434,986 includes a thermal imager that is mounted along a process line that transports an object having at least one recently created thermal seal.
Yet, in addition, PCT publication WO2007/147158 describes a pulsed thermography defect detection apparatus including active and passive infrared (IR) thermography for non-destructive testing (NDT) of powdermetallic (P/M) components for on-line and off-line inspection.
Yet, further, European patent application publication EP0355699 describes a method for inspecting leakage of a sealed container, the method being based on changing an internal pressure of a vacuum chamber and having a conductive material at least at a portion to be inspected in the chamber and determining any change in the amount of expansion after a time when the detected amount of expansion shows the maximum value, to thereby to find out defects in the sealed container.